Discovery of Oldest Human Fossil Fills Evolutionary Gap

A newly unearthed fossil from Ethiopia is altering the timeline of human evolution, pushing it back by nearly a half-million years.

Under a searing African sun on January 29, 2013, an international team of paleoanthropologists scoured a dusty plateau looking for clues to the origins of mankind. Aboveground, the surrounding Ethiopian landscape at the northern end of the Rift Valley looked barren, but the field team knew that a bounteous crop of fossils could lurk just beneath the surface. Just 40 miles away, an earlier generation of paleoanthropologists in 1974 had unearthed the partial skeleton of the famous “Lucy,” a 3.2-million-year-old hominid.

More than 40 years later, however, a large gap in the human family tree has remained. Researchers have discovered numerous fossils from Lucy’s Australopithecus afarensis species—ape-like, bipedal human ancestors approximately 4 feet tall—that are 3 million years or older. They have also found numerous skeletal remnants from multiple lines of the human genus Homo—of which Homo sapiens are the only remaining species—that are 2.3 million years old and younger. Little, however, has been discovered from that interim 700,000-year period to explain or date the evolution from Australopithecus to Homo that occurred during that timeframe, and that gap in the fossil record has hampered scientists’ understanding of human origins.

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Close up view of the mandible just steps from where it was sighted by Chalachew Seyoum, an ASU graduate student from Ethiopia. (Credit: Kaye Reed)

The international team of paleoanthropologists working in Ethiopia in 2013 sought to close that gap by uncovering fossil evidence of human ancestors. The researchers, co-led by Arizona State University professors, spent a decade surveying the area before beginning the process of fossil collection in 2012. They had expected to find more specimens of Lucy’s species, but what they discovered instead turned out to be more surprising.

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Chalachew Seyoum, an Ethiopian native and Ph.D. student at Arizona State, was scouring the edge of a barren ridge when something caught his eye. He immediately knew he had spotted something important in the eroded hillside and cried out to his fellow team members, who raced up the plateau. What Seyoum showed them gave them goose bumps. It was the well-preserved left side of a hominid’s lower jaw with five teeth attached. The fossil itself was small, but it turns out that its implications could be enormous.

According to a pair of papers published last week in the journal Science, the jawbone is the oldest fossil in the human ancestral line to have ever been found and dated, and it pushes back the timeline of human evolution by nearly a half-million years. The analysis of the lower jaw found more similarities with younger east African Homo specimens than older Australopithecus afarensis ones. The specimen’s primitive, sloping chin resembled that of Lucy, but its slimmer molars, symmetrical premolars and rounder, evenly proportioned jaw distinguished it as a member of the Homo genus. “Our detailed study of this specimen shows that it is more advanced, closer to humans, than previously discovered fossils in this area that date from around 3 million years ago back to about 3.5 million years ago,” paleoanthropologist Bill Kimbel, director of Arizona State’s Institute of Human Origins, said in the university-released video.

Radiometric testing of the layers of volcanic ash surrounding the fossil has revealed the approximate age of the jawbone to be between 2.75 and 2.8 million years old, which makes it the earliest evidence of the Homo genus ever discovered. Prior to the find, the oldest known Homo fossil had been a 2.3-million-year-old upper jaw also found in northern Ethiopia, so the dating pushes back the origin of the Homo genus by at least 400,000 years.

It is thought that at the time that the human ancestor lived, the section of Ethiopia in which it was found more closely resembled the Serengeti with open grasslands near lakes and rivers frequented by grazing animals such as gazelles and zebras. Research remains to be done, however, to discover what the specimen ate and whether it employed stone tools. More work needs to be done as well to determine whether it could be from a previously unknown human species or an extinct one such as Homo habilis.

The hope is that the fossil could serve as a crucial clue in the knowledge gap of evolutionary history. “The importance of the specimen is that it adds a data point to a period of time in our ancestry in which we have very little information,” Kimbel said. “This is a little piece of the puzzle that opens the door to new types of questions and field investigations that we can go after to try to find additional evidence to fill in this poorly known time period.”